Process for the fabrication of nitrogen containing pulverized carbon

ABSTRACT

A process for the fabrication of nitrogen containing pulverized carbon for incorporation thereof as an electrode material for electrochemical cells, particularly fuel cells. A carbonizable, nitrogen containing organic polymer is completely dissolved while being heated in a concentrated salt solution of an inorganic halides or thiocyanate. The resultant highly viscous fluid is carbonized at temperatures between 700* and 1200*C and the resultant product is crushed.

United States Patent [1 1- 252/422, 425, 447; 117/46 CC, 46 CH; 423/449Luft et a]. ['45] Aug. 28, 1973 PROCESS FOR THE FABRICATION OF [56]References Cited NITROGEN CONTAINING PULVERIZED UNITED STATES PATENTSCARBON 3,248,303 4/1966 Doying 252/425 x 75 Inventors: Gunter Lu, Lauf;Gerhard Richter; 3,297,490 1/1967 Barber et al. 136/122 r ard w nch,both o Er gen, Gentilhomme et al. x

ll f0 8 o "many Primary Examiner-L. Dewayne Rutledge [73] Assignee:Siemens aktlengesellschaft, Berlin A i ant E ami er-M. J. Andrews and iGermany Attorney- Arthur E. Wilfond, Herbert L. Lerner et al.

22 Fl 6: F b. 4 1972 l 1 e 57 ABSTRACT [21] Appl. No.: 223,548 A processfor the fabrication of nitrogen containing pulverized carbon forincorporation thereof as an elec- [30 Foreign Application p g, trodematerial for electrochemical cells, particularly Feb 22 Germany P 21 08417 3 fuel cells. A carbonizable, nitrogen containing organic polymer iscompletely dissolved while being heated in [52] cl 252/422 136/122423/449 a concentrated salt solution of an inorganic halides or [51] Intl 6 31/08 thiocyanate. The resultant highly viscous fluid is car- [58]Field of Search l lse/121 122- mind at temperatures between and and theresultant product is crushed.

6 Claims, 1 Drawing Figure 20'0 t [hi PROCESS FOR THE FABRICATIONQF'NITROGEN CONTAINING PULVERIZED CARBON.

tion, carbon maybe used, for instance, in fuel elements in whichhydrogen is used as afu'el gas, as a catalyzer carrier which containssmall quantities of catalyzer such as platinum or Raney-nickel, for thepurposeof oxidizing the hydrogen. Since, however, carbon demonstrates acertain activity in respect to oxygen reduction, carbon, for instance,may be, used asan electrode material for the-cathode in fuel elements inwhich oxygen or air is used as an oxidizing medium.

Catalyzersfor oxygen reduction, apart from the catalyzing activity-{mustfulfill other requiremehts'tl hey must be available insufficientquantities and at a reasonable price, and they shoulddemonstrate a good stability. Carbon, of course,is available insufficientquantity and at a favorable price; it is also a good conductorand the corrosion within the range of the oxygen potentials appears tobe tolerable; the activity, however, in connection with the oxygenreduction, is quite modest. Therefore, in many cases, additionalmetallic or oxydic catalizers are introduced into the-carbon. j 1 t Itis further known to activate carbon through the inclusion of nitrogen,whereby a noticeable improvement in its performance may be obtained. Theactivation may be obtained in a manner whereby carborr'is tempered in anammonia atmosphere. Nitrogen containing carbons may also be obtainedthrough the carbonization of materials such as gelatin,hexamethylentetramine,amines, urea and other animal products.

Although, during the disintegration of hydrogen peroxyde and also duringthe use for oxygen reduction in fuel elements with alkalineelectrolytes,these carbons demonstrate a useful catalytic activity, such electrodesdo not show a satisfactoryperformance during the oxygen reduction in.fuel elements with acidic electrolytes. Thus, particularly, in theknown carbonsduring long term experiments, the observed deactivation isvery troublesome. This deactivation, which hiis origin in the anodicdissolution of the carbon, may remain, however, within tolerable limitswith the use of carbon in fuel cell elements with alkalineelectrolytesand at room temperature. The deactivation, however, duringuse in practical, heavy duty fuel cell batteries} with acidicelectrolytes makes a disadvantageous andtroublesome appearance. Suchfuel cell batteries are operated, for reasons of water depletion andheatlrefmoval at temperatures of up to, 60C and higher, wherein thedeactivation may occur to" an extent which is noljohger tolerable."

In accordance with the invention, this is attained by completelydissolvinga carbonizable, nitrogen containing organic polymer, byheating in a concentrated salt solution. of an inorganic halides vorthioyanate, and in that the high viscosity liquid obtained'thereby iscarbonized at temperatures between 700 and 1 200C and i the resultingproductis then ground.

In anespecially advantageous embodiment of the process in accordancewith the invention, for instance,

polyacrylonitrile is added to a concentrated watery solution ofzinc'chloride in a proportion of the polyacryl' onitrile to the salt of1:1 to lzlO, preferably 1:2 to 1:5.

The resultant mixture' is heated to temperaturesof around l00 "C untilthe complete solution of the polyacrylonitrile is obtained and theresultant solution is heated in a nitrogen atmosphere with a heat-upspeed of l'to 10C per'minut'c, preferably 2 to 6C per minute, to atemperature of from 700 to l200C, preferably 900 to l000C, and whereinthe temperature is maintained at the final temperature till theconclusion of the carbonization. l I

Polyacrylonitrile is especially, suitable for carbonization, since itdoes-not melt and, during the thermal treatment, few tar-like productsare produced there'- from. During the .carbonization ofpolyacrylonitrile', a

better yield of carbon with good mechanical properties is obtained; Thecarbonization product of polyacrylonitrile shows a .high nitrogencontent which, however, varies with the carbonization temperature. Thehigh ni trogen content distinguishes the carbon obtained throughcarbonizationof polyacrylonitrile. also in respect to carbonizationproducts of other nitrogen containing organic substances, in which.relativelylittle nitrogenremains inthe end product.

I If polyacrylonitrile is-heated underargon for one hour tovarious-temperatures, the carbon formed It is the task of theinvention10' provide a method for v the production of nitrogen containingpulverized carbonas an electrode material for'electrochemical'cells,

particularly fuel cells, by'which carbon is obtained ,-in

thereby contains the following quantitiesof nitrogen:

TemperaturesinC 600 .700 soo soo .ioootloo Nitrogen content in Y Ypercentage of weight 17.9 17.0 14.7. 9.5 5.8 {I 3.6

Carbon which has been obtained through such a thermal treatment;however, demonstrates smallactivity and, furthermore, has anactivesurface which'is very small. Thus, .forinstance, forthecarbonobtained during the thermal treatment :at 800C; a BETsurface area,i. e.a surface area measured according to the method of Brunauer, Emmettand Teller (see: S.J.Gregg and K.S'.W.Sing, Adsorption, Surface Area andPorosity," Academic Press, London and New York,1l967, p. 35 andfollowing), of only about Strif/g has been determined.Thetotal'weightloss during the pyrolysis of polyacryloni'tn'le at a temperature ofup-to 1000C comes to'about percent. In spite of its otherwise goodcharacteristics, 'polyacrylonitrile therefore had to be considereduseless for the purpose mentioned her,einabove.' I

While it is known that, during the fabrication of activatedcarbon fromcellulose containing products such as wood flour, chemical activationmay be attained through soaking of the primary productswith a solutionof zincchloride, potassium sulfide and potassium thiocyanate-orphosphoric acid and sulfuric acid, effortsto obtain a correspondingactivation also withhydrophobic=polymers such aspolyacrylonitrile,,however., had to be considered as hopeless.

The invention makes use of the fact, that polyacrylonitrile is solublein certain salt solutions. Such solutions, for instance, becomeavailable when the polymerization of acrylnitrile into polyacrylonitrileis carried out in salt solutions. Similarly, this is also true for othernitrogen containing polymers. Surprisingly, from such solutions, inaccordance with the process of the invention, a carbon with very highactivity and a large surface may be obtained. This carbon furthermoreshows an improved conductivity and, moreover, an improved yield of thecarbon is obtained through a decreased formation of tarlike productsduring its manufacture. All of these positive effects have their originin the beneficial effect of the salts which have been added during themanufacture. I

When a carbonizable nitrogen-containing organic polymer, for instancepolyacrylonitrile, is completely dissolved under heat in concentratedsalt solutions of inorganic halides or thiocyanate, and the resultantsolutions are evaporated, highly viscous liquids are obtained which maybe carbonized into highly activated carbon with an active surface of1000 to 1400 m per gram. The carbonization temperature is preferablyselected to lie between 700 and 1200C, and is preferably located in therange between 900 and l000C.'The carbonization temperature is selectedin the indicated range in order to obtain a stable carbon with goodconduction capability.

The inorganic halides and thiocyanates, which are preferably used asconcentrated watery solutions, are used in the formof ammonium salts,alkaline salts, zinc or iron salts, such .as NH SCN, LiBr, NaSCN, KSCN,ZnCL or FeCl However, other solvent media may be used, for instancedimethylformamide for zinc chloride, and the salts could, with acorrespondingly lower melting point, also be used in the form of fusedsolutions with correspondingly lower melting points. The ratio of thepolymers to the salt may advantageously lie between 1:1 and 1:10, thepreferred ratio being between 1:2 and 1:5.

Carbon which has been fabricated by the process in accordance with theinvention shows a good output capacity for use as oxygen electrode infuel cell elements. An electrode, whichfor instance, contains only mg ofcarbon'per cm which has been obtained through the carbonization ofpolyacrylonitrile .in the presence of zinc chloride, delivers in l m H80 at a temperature of 60C a current density of 30 mA/cm at a potentialof 700mV, measured against the reversible hydrogen potential. Incomparison thereto, the best activated carbon taken from a series oftested carbons, available I commercially under the name of LEV 585, andunclerv the same operating conditions, could be loaded only to l/lO ofthis current density, namely to 3 mA/cm. The

activity of a carbon which has been made from polyacrylonitrile withoutaddition of zinc chloride during the carbonization, as alreadymentioned, is extremely small. Under the conditions mentioned above,carbon which has been made in this manner does not provide a measurablecurrent.

The process in accordancewith the invention may be furtheradvantageouslyenlarged in that the ground car- An additional activationof the carbon may advantageously be also obtained in that duringthedissolution of the nitrogen containing polymers in the salt solution,a soluble, nitrogen containing compound is added. Preferably, compoundsare used therefor, which contain more nitrogen than the polymers, forinstance melamine (2.4.6triamino-l.3.5-triazine), urea or urotro pine(hexamethylentetramine). Such compounds, just as ammonia, result in anincrease d nitrogen content of the final carbon. The increasednitrogencontent is of great significance, since the carbon thereby acquires agreater stability in respect to the oxygen; Furthermore, a highernitrogen content contributes substantially to the fact that the carboncrystal grid in the carbon is badly distorted and in that thecrystallites formed during its fabrication remain small. These effectsinfluence the electrochemical properties of carbon in a beneficialmanner. I

The method of the invention, together with additional objects andadvantages thereof will be best understood from the followingdescription:

The invention will be further explained with reference to severalexemplified embodiments and a FIG- URE.

Comparative Experiment a. Fabricationof carbon through the carbonizationof polyacrylonitrile without additives: 10 g of polyacrylonitrile inpowdereform are heated in a tubular oven in the presence of a current ofargon should at first be treated before the sedimentation in asedimentation fluid with ultrasound, in order to separate the individualparticles from one another and to facilitate their wetting thereby. Thediaphragm, together with the separated carbon is dried and assembledinto the known half cell arrangement in accordance with the principle ofthe supported electrode. For the purpose of reinforcement of theasbestos paper cover layer, that is of the diaphragm, an apertured sheetmetal of tantalum is used on the side of the electrolyte and a net madeout of.polypropylene. The contact and support of the powdery materialson the gas side is attained by means of a carbon web and a gilded,perforated tantalum sheet.

c. Electrochemical Measurements:

The electrode obtained in the .above manner, with a.

coating of 20mg carbon per cm, is maintained potentiostatic at 60C in lm H 80 under an oxygen pressure of 210 Film at a potential of 700 mV,measured I against the reversible hydrogen electrode. At this pobon isadditionally activated in an ammonia streams, This may be carried out ina known manner by means 7 of heating in an ammonia atmosphere.-

tential, no measurable reduction current is observed. Only at potentialswhich are more negative than 500 mV, a measurable cathodic. current isflowing. At a potential of 500 mV, the current density comprises 3-mA/cm.

EXAMPLE 1 a. Fabrication of nitrogen containing, powdery carbon:

g polyacrylonitrile are mixed with 50 mlof a watery solution of 1000 gzinc chloride per liter, and totally dissolved through heating of themixture to 100C. The completely clear and viscous solutionis introducedinto a quartz boat and is heated in a tubular oven in a nitrogenatmosphere with a heat-up rate of 2C per minute up to 1 000C andmaintained for one hour at this temperature. The carbon obtainedthereby, is crushed and'screened out as in the comparison experiment.

b. Electrochemical Measurements:

The powdery carbon, which has been fabricated in accordance with examplela, is included into an electrode, as in the comparison example. Themeasurements undertaken on this electrode, under the sameconditions asin the comparison example, provide a current of 30 mA/cm at a potentialof 700 mV. The long term behavior of such an electrode will beconsidered separately.

Comparative measurements of commercially available activated carbonsprovidedonly low current densities. The best behavior out of the seriesof tested carbons was shown by an*'activatedcarbon, commerciallyavailable under thenat'ne of Lev 585, which, under the circumstances inaccordance with the comparison ex ample, provided a current density of 3mA/cm. The current densities which were measured for theother carbonswere found to be below this value.

Samples of glucose, treated in accordance with example la, providedcarbons which, after the inclusion into electrodes, did not delivermeasurable current, either.

EXAMPLE 2 10 g of polyacrylonitrile were mixed with 100 ml of a waterysolution, which contained 500 g zinc chloride 7 per liter. After heatingof the mixture for one hour. up to 100C, the polyacrylnitrile hadcompletelydissolved. The carbonization was then achieved, as describedin Example 1. The resultant carbon, corresponding to Example 1, was thenincluded into an electrodeand furnished, under the conditions giventherein, a current density of 25 mA/cm.

EXAMPLE 3 EXAMPLE 4 4 g of carbon, made in accordance with Example 1,are heated in a tubular oven in the presence of an ammonia atmosphere inone hour up to 850C and held at chemical measurements, corresponding tothose of the this temperature for three hours. 350 mg of powdery carbonactivated in this manner is assembled into an comparison example,furnish, at'an'oxygen pressure of 2.6 10 Nlm -an'd a coating of 30mg/cm, a current density of mA/cm The current-time curve will beconsidered separately. 1

For'p'urposes of comparison, 5 g "of the carbon, commercially availableunder the name'of Lev 585, was activated with ammonia under the sameconditions. An electrode comprised of this caibonjfurnished under theconditions given in the comparison example, an initial current densityof 48 mA/cm. The current timecurve will also be considered separately.

EXAMPLE 5 5 g of powdery polyacrylonitrile are dissolved while beingheated in 40 ml of a watery zinc chloride solution, which contains 1,000g zinc chloride per liter in solution. While stirring, 4 g of melamineare added to this solution. The mixture obtained thereby is heated,while stirring, to a point of boiling, until a complete dissolution hastaken place. The clear solution is heated to .1000C in a tubular ovenwithin three hours. During this process, the arrangement isflushedwithpurified nitrogen up to a temperature of 700C. In the temper'atuterangeof 700 to 1000C, the supply of nitrogen is turned off. The .testsample is subsequently kept for one,

hour at a temperature of 1000. The resultant carbon is crushedin amortar, screened,:and a portion with a particle size'ofunder 32 p. isused for'the fabrication of an electrode. 3

The electrode, made as inthe comparison example and which has beentestedunder the same conditions,

supplies a current density of about 51 mA/cm. In such a carbon, at thebeginning of the-loading, the current does not fall off as much as withsimilarly fabricated carbons which, however, have been activated withammonia.

The FIGURE in graphical representation shows current-time curves ofdifferent electrodes. The time t is shown as an abscissa in hours andthe current density i as the ordinate in mA/cm The electrodes havingacoating of 20 or, as the case may be, 30 mg/cm The measurements wereundertaken in 1 m H 80 at a temperature of 60C. The oxygen pressure was2 10 N/m, the polarization 700 mV, measured against the reversiblehydrogen potential. 5

Graph 1 shows the behavior of an electrode which is comprised ofactivated carbon, obtainable commercially underthe name of Lev 585,which was subject to the activation in an ammonia currenL The currentdensity with a coating of 20 mg/cmfliwhich initially was 48 mA/cm, fallsoff relatively fast and stabilizes at a value of about 8 mA/cm'. Curve 2shows the behavior of an electrode made as in Example 1, which, inaccordance with the process-of the invention, has been obtained throughcarbonization of polyacrylonitrile. The current density (coating 20mg/cm'lamounts, at the beginning of the test, to 30 mA/cm, and afterabout hours, stabilizes at a value of about 13 mA/cm. Curve 3 shows thebehavior of an electrode consisting of car bon, made as in Example 4,that is, by carbonization of polyacrylnitrile by the process inaccordance with the invention and subsequent activation with ammonia.'

With a coating of 30 mg/cm, a current density of 80 mA/cm is obtained atthe beginning of the measurement, which stabilizes after 150 hours at avalue of about 29 mA/cm.

Graph 4 shows the behavior of an electrode consisting of carbon, made asin Example 5, that is, through carbonization of polyacrylonitrile, inaccordance with the process of the invention, with the addition ofmelamine. This electrode show a relatively constant long term behavior.After an initial current density of 51 mA/cm, the current densitystabilizes after 170 hours at the relatively high value of about 37mA/cm (coating: mg/cm).

It is thus clearly evident from these curves that car bon, made inaccordance with the process of the invention, shows, in electrodes, asmaller deactivation than electrodes made from commercially availablecarbons.

In the process in accordance with the invention, for the purpose ofmaking nitrogen containing powdery carbon, in addition topolyacrylonitrile, other carbonizable nitrogen containing organicpolymers may also be used, for instance, polyethylenimine. Proteins,also, may be used for the fabrication of powdery, nitrogen containingcarbon. However, the activation and stability of such carbons is not asgood as that of carbons which have been made from polyacrylonitrile andcorresponding polymers. Carbon made in accordance with the process ofthe invention is not limited to the application in electrochemicalcells. Such carbon may also for auto-oxidation reactions.

Although the invention is illustrated and described herein as processfor the fabrication of nitrogen containing pulverized carbon, it isnevertheless not intended to be limited to the details shown, sincevarious modifications may be made therein without departing from thespirit of the invention and within the scope and range of equivalents ofthe claims.

' be used for other purposes, for instance, as catalyzer It is claimed:

1. Process for making an activated pulverized carbon for use aselectrode material, comprising the steps of mixing a carbonizable,nitrogen containing organic polymer into a concentrated inorganic saltsolution selected from the group consisting of halides and thiocyanates,heating said mixture until the polymer has completely dissolved;carbonizing the resultant viscous fluid at temperatures between 700 andl200C, and crushing the resultant carbonaceous product.

2. Process in accordance with claim 1, wherein polyacrylonitrile isadded to a concentrated watery solution of zinc chloride in a ratio ofthe polyacrylnitrile to the salt of 1:1 to 1:10, the mixture is heatedto a temperature of C to a complete dissolution of the polyacrylnitrile,and the resultant solution is heated in a nitrogen atmosphere at aheating rate of 1 to 10C per minute up to 700 to l200C, the temperatureis maintained at the value of the final temperature until attainment ofcarbonization.

3. Process in accordance with claim 2, wherein said ratio of thepolyacrylonitrile to the salt is between 1:2 to 1:5, the heating rate is2 to 6C per minute, and the solution is heated to between 900 to 1000C.

4. Process in accordance with claim 1, including the step of activatingthe crushed carbon in an ammonia stream.

5. Process in accordance with claim 1, including the step of adding asoluble nitrogen-containing compound, during the dissolution of thenitrogencontaining polymer in the salt solution, whereby an additionalactivation of the carbon is effected.

6. The process according to claim 5, wherein said added compoundcomprises melamine.

2. Process in accordance with claim 1, wherein polyacrylonitrile isadded to a concentrated watery solution of zinc chloride in a ratio ofthe polyacrylnitrile to the salt of 1:1 to 1:10, the mixture is heatedto a temperature of 100*C to a complete dissolution of thepolyacrylnitrile, and the resultant solution is heated in a nitrogenatmosphere at a heating rate of 1* to 10*C per minute up to 700* to1200*C, the temperature is maintained at the value of the finaltemperature until attainment of carbonization.
 3. Process in accordancewith claim 2, wherein said ratio of the polyacrylonitrile to the salt isbetween 1:2 to 1:5, the heating rate is 2* to 6*C per minute, and thesolution is heated to between 900* to 1000*C.
 4. Process in accordancewith claim 1, including the step of activating the crushed carbon in anammonia stream.
 5. Process in accordance with claim 1, including thestep of adding a soluble nitrogen-containing compound, during thedissolution of the nitrogen-containing polymer in the salt solution,whereby an additional activation of the carbon is effected.
 6. Theprocess according to claim 5, wherein said added compound comprisesmelamine.